where \(R_{d}\)  is the ratio of the baryon to photon energy density at the drag epoch, \(\mathrm {k_{eq}}\)  is the wave-number at the equality epoch
radiation-matter, \(r_{d}(z_{d})\)  is the sound horizon at the drag epoch, \(G\)  is a factor of suppression, \(\mathrm {a_{*}}\)  represents the scale factor at
the recombination epoch, \(\mathrm {a_{eq}}\)  represents the scale factor at the equality epoch radiation-matter and \(\mathrm {k_{silk}}\)  represents the
Silk damping scale [1]}, [2]}.
